Aerogels are super porous materials having a porosity of up to about 99% and a mesopore size of 50 nm or less, and exhibit lighter and better characteristics in terms of super heat-insulation/super lightness/super porosity/super low-dielectric properties than any other materials which have been developed to date. As a result, aerogels are spotlighted as fantastic materials which have broad applicability in energy/environment/electric and electronic fields as well as super heat-insulation materials. In particular, aerogels have a three-dimensional super porous network structure, pores of which have a smaller size than an average free path of air molecules. As a result, aerogels provide various merits, such as minimization of thermal conduction, avoidance of sound wave transfer, sunlight spreading, and hydrophobic properties, thereby enabling a wide range of applications in construction and other industries.
Aerogels have a thermal conductivity of about 0.025 W/m·K or less, which is much lower than any other known materials. As such, since silica aerogels exhibit such a low thermal conductivity, it has increasingly attracted attention towards application to LNG carriers, refrigerators, freezing machines, heat condensers, and the like. Silica aerogels are considered important materials not only in view of thermal conductivity but also in view of environment, since the silica aerogels exhibit superior heat insulating performance to polyurethane foam made of CFC causing destruction of the ozone layer or other toxic fibrous inorganic heat insulating materials.
Radiation heat transfer of aerogels is generally governed by absorption of infrared light. That is, at a low temperature of 20° C. or less, radiation heat transfer may easily occur by allowing infrared light at wavelength of 30 μm or more to be transmitted through the aerogel, and, at normal temperature and high temperature, the heat transfer may easily occur by allowing infrared light in the wavelength band of 2˜8 μm to be transmitted through the aerogel. As a heat insulating material, the silica aerogel has a merit of visible light transparency, which allows the silica aerogel to be applied to windows or skylights. However, the aerogel also allows transmission of infrared light, particularly, in the wavelength band of 2˜8 μm. Accordingly, although the aerogel does not cause any specific problem at low temperature due to low contribution of radiation heat transfer, it is necessary to minimize radiation heat transfer at high temperature in order to use the aerogel as a high temperature heat insulating material, since the radiation heat transfer is the main heat transfer mechanism of the aerogel at high temperature. As a method for reducing high temperature radiation heat transfer, opaque silica aerogel granules are prepared by adding an opacifier capable of absorbing infrared light at wavelengths of 8 μm or less to the silica aerogel.
Unlike silica aerogel granules, silica aerogel powder is prepared by placing an opacifier containing silica sol in a container or frame, followed by gelation and pulverization, thereby causing separation of the opacifier having a size of several micrometers or more from the silica gel. Further, even when the aerogel powder is directly prepared by adding an opacifier to a silica sol and is subjected to gelation, there is a problem of separation of the opacifier from the silica gel. Thus, the opacifier is not added in preparation of the silica aerogel powder. In addition, although the silica aerogel powder has thermal conductivity similar to that of the silica aerogel granules at room temperature, the silica aerogel powder has a particle size of about 10˜20 μm and a very light weight, and is thus easily blown by wind, thereby causing difficulty in filling and handling of the powder. Moreover, the silica aerogel powder exhibits large heat radiation at high temperature, thereby limiting the applications range thereof.
In preparation of aerogel, it is an essential technique to dry the aerogel without contraction while maintaining a porous structure thereof. Generally, the overall process for aerogel preparation may be divided into a sol-gel process in which a gel is prepared from a sol, surface modification for hydrophobication in which the interior and surface of the gel are substituted by a solvent for modification, and a drying process. When evaporating the solvent in air during drying of the wet gel, the wet gel is likely to undergo contraction and cracking caused by differences in capillary force and solvent extraction rate at an air/liquid interface during the drying process, and a super-critical drying process is conventionally used to suppress such phenomena. However, supercritical drying is performed at high pressure and increases manufacturing costs, thereby providing an obstacle in commercialization of aerogels. Therefore, there is a need for development of a new ambient pressure drying process capable of solving problems of the supercritical drying process in terms of economic feasibility, safety, continuity, and the like. Further, in order to prepare silica aerogels at low cost, it is necessary to develop a process enabling continuous mass production of silica aerogels using inexpensive water glass and surface modifiers as starting materials instead of expensive alkoxide-based materials.
In the related art, when preparing silica aerogel granules using water glass, sodium components are generally removed from a water glass solution using an ion exchange resin to prepare a silica sol, which in turn is used to prepare granules. In addition, for gelation of the sol, a wet gel is produced by adding a basic substance to the silica sol and spraying liquid droplets of the silica sol into a non-polar organic solvent, or by dropping or spraying the silica sol into the non-polar organic solvent to which the basic substance has previously been added, followed by hydrophobication of the alcohol gel through surface modification and solvent substitution using an organic silane compound, and supercritical drying (U.S. Pat. No. 6,197,270). Further, as starting materials, expensive materials such as silicon alkoxide, alkyl silicate and alkoxy silane are used instead of water glass, and surface modification for hydrophobication and super-critical drying are sequentially performed (Japanese Patent Laid-open Publication No. H08-15120). As such, the silica aerogel granules are generally prepared using water glass, silicon alkoxide, and the like through supercritical drying.
Further, in ambient pressure drying, water glass is used as a starting material for cost reduction. The known process for preparing silica aerogel using water glass was very complicate and was not so productive and cost-effective because it is necessary to use cation exchange resins for removal of sodium ions from the water glass containing sodium components as impurities, a basic substance for inducing gelation through hydrolysis and polymerization, and various additives such as a dispersant for facilitating dispersion of silica gel in a non-polar organic solvent, a surface modifier, a substitution solvent, and the like in the separate steps.